*Looks at his feet....then his hands...* i seem to be missing both. No really is this an output protection device? I want to protect my amps but i don't want a super complicated circuit just something so my amp won't overdrive itself if i turn the volume up too far.

I guess --- since folks are trying to connect more external devices via the serial or parallel port you don't want voltages or overloads on these ports. Instead of a card for these ports they are incorporated right into the motherboard. Thus, you can fritz the entire thing with an over-voltage, or too much current draw. There was a design in one of the popular US DIY electronics magazines using HP Opto's and some quad comparators a couple years ago. Separate grounds for both sides.